Nuclear magnetic resonance imaging (MRI) is one of the most useful diagnostic tools in medicine today. The technique is based on the discovery that atomic nuclei possess a magnetic moment which can be detected using a magnetic field. When placed in a magnetic field, nuclei arrange themselves in a manner that aligns their magnetic moments either with or against the direction of the magnetic field. Those that align against the field (so-called .beta.-spin nuclei) are higher in energy than those aligned with the field (so-called .alpha.-spin nuclei). The energy difference between .alpha.- and .beta.-spin nuclei is directly proportional to the strength of the applied field. To measure this difference in energy, the nuclei are subjected to electromagnetic radiation. Low energy .alpha.-spin nuclei absorb electromagnetic radiation causing them to "flip" against the field (or out of alignment with the field) into a high energy .beta.-spin. The frequency of the absorbed radiation, known as the "resonance frequency," reveals the .alpha.-.beta. energy difference. Alternatively, high energy .beta.-spin nuclei or non-aligned nuclei may "relax" into a low energy .alpha.-spin state with concomitant release of electromagnetic radiation. Both the resonance frequency and the relaxation rates (measured in terms of relaxation rate (1/T.sub.1) wherein T.sub.1 =the time that it takes for non-aligned nuclei to relax) for any nucleus is dependent on the magnetic environment surrounding that nucleus. For example, nuclei within a substantial (strong) electron shell are shielded from external magnetic fields and as a result possess a smaller .alpha.-.beta. energy difference, compared to nuclei within a weak electron shell. Also, nuclei situated in the proximity of other paramagnetic nuclei relax faster and have higher relaxation rates than those for which such a relaxation mechanism is not available.
It is the difference in the nuclear magnetic relaxation rates of water protons that is most commonly used as the source of information in diagnostic MRI. Water is ubiquitously found in soft tissue. The hydrogen atoms in water possess paramagnetic nuclei (protons) that, when subjected to resonance frequency and relaxation-rate measurements, provide direct information regarding their physical microenvironment. The information can then be processed through computational techniques to obtain detailed anatomical images. Inspection of these images can reveal the presence of diseased or abnormal tissue, lesions and fractures or other malfunctions that may be present.
Not surprisingly, MR imaging of some tissue is more difficult than others. This may be due to any number of factors which result in low contrast and hence lack of resolution in the MR images obtained. Previous efforts to overcome such problems have led to the discovery of so-called contrast enhancement agents. Typically, such agents contain a paramagnetic metal ion which is capable of altering the relaxation rates of water protons in its proximity. Upon administration to a patient, the metal-containing contrast agent is absorbed by various organs depending on the patient's metabolic and excretion pathways. Once absorbed, the contrast agent alters the relaxation rates of water protons in the organ or tissue in which it resides. The MR images of that organ or tissue thus achieve enhanced contrast with respect to neighboring tissue which contains lower concentrations or none of the paramagnetic agent.
Because of the acute toxicity of most paramagnetic metals, however, ordinary inorganic salts of paramagnetic metals are unsatisfactory as contrast agents. A solution to this problem is to use an organic chelating ligand or metal-sequestering agent. Through complexation to the metal, the organic chelating ligand would prevent release of free, toxic metal yet allow proton relaxation enhancement by acting as a non-toxic paramagnetic carrier.
To be effective, MRI contrast agents (paramagnetic metal-ligand complexes) must satisfy several criteria. They must be stable and have high formation constants so that release of the toxic metal is prevented. They must be sufficiently soluble in aqueous solutions to facilitate their administration to a patient. And, they must be capable of efficient enhancement of relaxation rates of water protons in solution. Efficiency is generally measured in terms of "relaxivity" which is defined as the increase in relaxation rate per concentration of the paramagnetic complex measured in units of mM.
Gries et al. have described complexes for use as diagnostic agents in U.S. Pat. No. 4,647,447. Also, the active paramagnetic ingredient of the FDA-approved MRI contrast agent Magnevist.RTM. is a complex of diethylenetriaminepentacetic acid and gadolinium (III). In U.S. Pat. No. 4,899,755, Lauffer and Brady describe how paramagnetic metal-ligand complexes can be designed and synthesized to target specific tissue for MRI enhancement. This tissue-specific approach provides several improvements over the previously reported non-specific methods. Qualitatively, tissue-specific agents provide better MR images of the targeted tissues such as the liver and the bile duct. Quantitatively, tissue-specific contrast agents can be used in lower concentrations to achieve image enhancements similar to that observed with non-specific agents at higher doses. Thus, previously undetectable (or difficult to detect) liver tumors or malfunctions in the biliary system can be detected using hepato-biliary-specific contrast agents.
Another improvement upon the non-specific methods of the art is provided in U.S. Pat. No. 4,880,008 where the relaxivity-enhancement of contrast agents is shown to improve upon noncovalent binding to specific tissue proteins. This strategy not only allows targeting of specific tissue containing the binding proteins, it also provides image enhancement at even lower doses than previously possible (due to improved relaxivity enhancement of the protein-bound agents).
In spite of these advances, there remains a need for MRI contrast agents that combine favorable stability, solubility, relaxivity-enhancement and protein-binding characteristics with a tissue-specific pharmaco-kinetic profile.